JP5331051B2 - Light emitting element - Google Patents

Description

  The present invention relates to a light emitting element, and more particularly to a light emitting element used in a backlight light source device and the like.

  In recent years, the market of liquid crystal display devices using liquid crystal panels as display devices such as flat-screen televisions has been rapidly growing. The liquid crystal display device includes a liquid crystal panel as a transmissive light modulation element, and a light source device that is provided on the back surface of the liquid crystal panel and emits light to the liquid crystal panel. The liquid crystal panel forms an image by controlling the transmittance of light emitted from the light source device. While cold cathode fluorescent lamps (CCFLs) have been used as light sources for light source devices, in recent years, LED light source devices using LED (Light Emitting Diode) elements have been developed due to the trend of energy saving. LED light source devices using LEDs as light sources can be mainly classified into two types. The first is a direct type in which LED elements are arranged two-dimensionally directly behind the display screen, and the second is the arrangement of the LED elements on the side of the liquid crystal panel and using the light guide plate from the back of the liquid crystal panel. This is an edge light type that emits light. At present, the LED light source device is mainly the direct type, but the development of the edge light type is progressing with the demand for thinning of the liquid crystal display device.

  A conventional LED element for liquid crystal display has a configuration in which a yellow phosphor having a center wavelength of about 570 nm is coated so as to cover an LED chip that emits blue light having a center wavelength of about 440 nm. The LED chip is driven to emit blue light, and the emitted blue light is absorbed by the phosphor to emit yellow light. Since blue and yellow are complementary colors, an LED element that functions as a white light source can be realized.

JP 2009-158274 A

  However, when the conventional LED element is used as an LED light source device of an edge light type liquid crystal display device, the light emitted from the LED element cannot be efficiently incident on the light guide plate, and the use efficiency of the light emitted from the LED element is poor. There is a problem. In order to improve the incident light rate to the light guide plate, a method of covering the surface of the LED element with a scattering lens that is a cylindrical lens is disclosed (for example, refer to Patent Document 1). However, in this case, there arises a problem that the thickness of the light guide plate cannot be reduced. The emission angle of light emitted from the surface of the LED element is a so-called Lambertian, and light having a full width at half maximum of 120 ° is emitted. In order to efficiently collect the emitted light having such radiation characteristics by the lens, the size of the lens needs to be 5 to 10 times that of the LED element. Since the size of the LED element is about 0.5 mm × 0.5 mm, the size of the lens needs to be about 2.5 mm to 5 mm. On the other hand, in order to efficiently guide light to the light guide plate, it is necessary to increase the thickness of the light guide plate to the size of the lens. Therefore, the thickness of the light guide plate needs to be about 2.5 mm to 5 mm, which is a limitation for reducing the thickness of the liquid crystal panel.

  Moreover, since the light emitted from the CCFL and the LED chip is spontaneous emission light, the polarization direction is random. Since the transmittance of the liquid crystal panel is controlled by using polarized light, the liquid crystal panel is provided with a polarizing plate on the light incident side so that only specific polarized light that is required is incident on the liquid crystal. Specifically, light having a different angle from the required polarization direction is absorbed or reflected by the polarizing plate. The transmittance of the polarizing plate is approximately 100% for the required polarized light, and approximately 0% for the polarized light having an angle different from the required polarization direction by 90 degrees. The transmittance of light having a polarization angle during this period is cos θ × 100%, where θ is the angle with respect to a specific polarization direction. When the polarization direction is random, only about 50% of the light incident on the polarizing plate passes through the polarizing plate and enters the liquid crystal panel. Since 50% of the light generated by the light source device is removed by the polarizing plate and used for liquid crystal display, the light utilization efficiency is 50% at the maximum. As described above, there is a problem that the same energy as the light energy used for the liquid crystal display is not effectively used.

  An object of the present invention is to solve the above-described problems and to realize a light-emitting element with high utilization efficiency of emitted light when used in a light source device.

  Specifically, a light emitting device according to the present invention is formed on a main surface of a substrate, and has a semiconductor multilayer film having an active layer that generates light having a first wavelength, and is formed on the semiconductor multilayer film. And a plurality of phosphor layers constituting the two-dimensional periodic structure, wherein the phosphor layers are excited by light of the first wavelength to generate light of the second wavelength, and the semiconductor multilayer film has the first The light having a wavelength waveguide and a light having a second wavelength is guided, and the light emitted from the end face of the optical waveguide is light in a direction horizontal to the direction in which the direction of the electric field is perpendicular to the main surface. The percentage of is high.

  Since the light-emitting element of the present invention can confine light having the first wavelength and light having the second wavelength in the optical waveguide, the vertical emission angle and the horizontal emission angle can be reduced. Therefore, it is possible to efficiently couple with the light guide plate or collimate efficiently with a small lens. As a result, the light use efficiency can be improved.

  In the light emitting device of the present invention, the first two-dimensional periodic structure forms a photonic band gap with respect to light having a direction of an electric field perpendicular to the main surface of light having the second wavelength. Also good. With such a configuration, the light having the second wavelength does not have a mode in which the direction of the electric field is a direction perpendicular to the main surface of the substrate. For this reason, only spontaneous emission light and stimulated emission light whose electric field direction is parallel to the main surface of the substrate are generated inside the optical waveguide. As a result, a light emitting element that emits light in a specific polarization direction can be realized.

  In the light emitting device of the present invention, the phosphor layer formed in the central portion of the optical waveguide among the plurality of phosphor layers constitutes a first two-dimensional periodic structure, and the optical waveguide among the plurality of phosphor layers. The phosphor layer formed on the outer edge portion of the first layer constitutes a second two-dimensional periodic structure, and the first two-dimensional periodic structure and the second two-dimensional periodic structure form a basic unit that forms a period or a periodic structure. May have different sizes or shapes. In this case, the second two-dimensional periodic structure may form a photonic band gap with respect to light of the second wavelength light whose electric field direction is parallel to the main surface. By adopting such a configuration, it becomes possible to more efficiently confine light having the second wavelength, which is TE-polarized light, in the optical waveguide.

  The light emitting device of the present invention may further include a transparent electrode formed between the semiconductor multilayer film and the phosphor layer.

  According to the light emitting element of the present invention, a light emitting element with high utilization efficiency of emitted light can be realized when used in a light source device.

It is a perspective view which shows the manufacturing method of the light emitting element which concerns on one Embodiment to process order. It is a top view which shows the two-dimensional periodic structure of a fluorescent substance layer. The operation principle of the light emitting element concerning one embodiment is shown, (a) is a sectional view in an end face, and (b) is a sectional view of a direction along an optical waveguide. It is a figure which shows the photonic band structure of the photonic crystal formed of the fluorescent substance layer. (A) And (b) is a figure which shows the light distribution in the semiconductor multilayer film in wavelength 440nm and wavelength 570nm, respectively. It is a top view which shows the modification of the two-dimensional periodic structure of a fluorescent substance layer. It is a figure which shows the photonic band structure in the fluorescent substance layer formed in the outer edge part of an optical waveguide. It is a figure which shows the example which used the light emitting element which concerns on one Embodiment for the backlight of a liquid crystal panel. It is a figure which shows the example which used the light emitting element which concerns on one Embodiment for the light source of a projector.

First, a configuration of a light emitting device and a manufacturing method thereof according to an embodiment will be described with reference to the drawings. First, as shown in FIG. 1A, a nitride semiconductor is formed on a substrate 101 made of n-type GaN having a (0001) plane of the main surface by a metal organic chemical vapor deposition (MOCVD) method or the like. A semiconductor multilayer film 102 made of is formed. The semiconductor multilayer film 102 includes, for example, an n-type cladding layer 121, an active layer 122, a p-side light guide layer 123, an electron overflow stop layer (OFS layer, not shown), and a p-type contact layer 125, which are sequentially formed from the substrate 101 side. And it is sufficient. The n-type cladding layer 121 may be made of n-type Al _0.8 In _0.2 N having a thickness of 1.6 μm and a silicon (Si) concentration of 5 × 10 ¹⁷ cm ⁻³ . The active layer 122 may have a double quantum well structure in which a well layer made of In _0.25 Ga _0.85 N with a thickness of 3 nm and a barrier layer made of undoped In _0.03 Ga _0.97 N with a thickness of 7 nm are stacked. In this case, the emission wavelength is about 440 nm. The p-side light guide layer 123 may be undoped In _0.02 Ga _0.98 N having a thickness of 50 nm. The OFS layer may be p-type Al _0.2 Ga _0.8 N having a thickness of 10 nm and an Mg concentration of 1 × 10 ¹⁹ cm ⁻³ . The p-type contact layer 125 may be p-type GaN having a thickness of 50 nm and an Mg concentration of 3 × 10 ¹⁹ cm ⁻³ . These compositions, film thicknesses, and the like are examples and may be changed as appropriate.

Next, as shown in FIG. 1B, the current confinement layer 103 and the transparent electrode 104 are formed on the semiconductor multilayer film 102. The current confinement layer 103 has a width that exposes the p-type contact layer 125 after depositing a silicon oxide film (SiO ₂ film) having a thickness of 100 nm on the semiconductor multilayer film 102 by a chemical vapor deposition (CVD) method or the like. May be formed by wet etching or the like. The transparent electrode 104 may be formed by sputtering or the like with an indium tin oxide (ITO) film having a thickness of about 100 nm so as to cover the current confinement layer 103 and be in contact with the p-type contact layer 125 in the opening. In the case where the light-emitting element is a super luminescence diode (SLD), the stripe direction may be inclined by about 10 ° with respect to the m-axis ([10-10]) of the substrate 101 made of GaN.

  Next, as shown in FIG. 1C, a plurality of phosphor layers 105 made of yttrium aluminum garnet (YAG: Ce) activated by cerium are formed. The phosphor layer 105 may be formed by depositing a phosphor having a thickness of about 100 nm by sputtering or the like and then using lithography such as electron beam exposure and dry etching. Each phosphor layer 105 may be, for example, a cylindrical shape having a diameter 2r of 128.5 nm and a triangular lattice shape having a period a of 257 nm. Further, as shown in FIG. 2, the M points are arranged in the first Brillouin zone so as to coincide with the stripe direction.

  Next, as shown in FIG. 1D, a p-electrode 107 and an n-electrode 108 are formed. If the p-electrode 107 is a laminated film (Ti / Al / Pt / Au) of titanium (Ti), aluminum (Al), platinum (Pt) and gold (Au) selectively formed on the transparent electrode 104. Good. After the n-electrode 108 is thinned so that the substrate 101 can be easily diced, Ti / Al / Pt / Au may be formed on the back surface of the substrate 101.

  Although FIGS. 1A to 1D illustrate one light emitting element, in practice, after a plurality of light emitting elements are formed on the wafer, the m-plane which is the (10-10) plane of the wafer is exposed. A primary cleavage and a secondary cleavage that exposes the a-plane which is the (11-20) plane are performed to make individual pieces.

  As for the chip size of the light emitting element, the chip width may be 200 μm and the chip length may be 800 μm including the bonding pad region (not shown).

  Hereinafter, the operation of the light emitting device of this embodiment will be described with reference to FIG. FIG. 3A shows a cross-sectional configuration in the direction perpendicular to the stripe, and FIG. 3B shows a cross-sectional configuration in the direction along the stripe.

Holes are injected from the p electrode 107 into the active layer 122 through the transparent electrode 104 and the p-type contact layer 125, and electrons are injected from the n electrode 108 into the active layer 122 through the substrate 101 and the n-type cladding layer 121. . Holes and electrons recombine almost immediately below the portion of the active layer 122 where the current confinement layer 103 is not formed, whereby blue spontaneous emission light having a wavelength of about 440 nm is generated. The refractive index of the transparent electrode 104 made of ITO is 2.1, and the refractive index of the current confinement layer 103 made of SiO ₂ is 1.46. Therefore, the transparent electrode 104 having a high refractive index serves as a loading layer, and the optical waveguide 109 is formed. Spontaneous emission light combined with the waveguide mode of the optical waveguide 109 propagates inside the optical waveguide 109.

  Increasing the voltage applied between the p-electrode 107 and the n-electrode 108 increases the density of carriers injected into the active layer 122. When the carrier density exceeds the transparent carrier density, stimulated emission by the active layer 122 is started, and the guided light is amplified. By making the active layer 122 have a quantum well structure, the direction of the electric field is higher than that of the TM polarized light in which the direction of the electric field is guided in the stacking direction of the semiconductor multilayer film 102, that is, the direction perpendicular to the main surface of the substrate 101. The optical amplification factor (optical gain) of TE-polarized light, which is guided light in a direction parallel to the main surface of the, becomes high. For this reason, in the optically amplified guided light, there are more TE polarized light than TM polarized light. Specifically, the TE polarization ratio (TE polarization / TM polarization) is greater than 15.

  Positive feedback of optical amplification occurs due to end face reflection, and laser oscillation occurs when the optical gain exceeds a threshold value. In this embodiment, the stripe forming the optical waveguide is inclined by 10 ° with respect to the m-axis. For this reason, the reflectivity (mode reflectivity) of the guided light with respect to the end face of the optical waveguide is reduced, and laser oscillation is suppressed. Therefore, a super luminescence diode having low coherence and low speckle noise is formed.

  The phosphor layer 105 made of YAG: Ce absorbs blue light that is amplified and propagated. When the YAG matrix doped with Ce absorbs blue light, excitons are generated and energy is transferred to Ce, which is the emission center. For this reason, yellow light having a wavelength derived from Ce of about 570 nm is generated.

  The phosphor layer 105 has a two-dimensional periodic structure and functions as a two-dimensional photonic crystal in the emission of excitons. FIG. 4 shows a result of theoretical calculation of a photonic band structure for light having a wavelength of λ in a vacuum in a two-dimensional photonic crystal by a plane expansion method. ω is the frequency of light, and c is the speed of light in a vacuum. In the calculation, the refractive index of the phosphor layer 105 is 2.0, the value r / a obtained by dividing the radius r of the phosphor layer 105 by the period a is 0.25, and the refractive index is between the phosphor layers 105. 1 was filled with air. In FIG. 4, the horizontal axis is the position on the line from the Γ point in FIG. 2 through the M point and the K point to the Γ point again.

  As shown in FIG. 4, a photonic band gap for TM polarized light exists in the range where a / λ is about 0.4 to 0.5. For this reason, when a is 257 nm, light having TM polarization is not generated from excitons in the range of λ from 514 nm to 642 nm. Therefore, the phosphor layer 105 emits only TE-polarized yellow light as fluorescence.

  As described above, the light emitting element of the present embodiment generates blue light and yellow light having a high TE deflection ratio, and thus functions as a white light source having a high TE deflection ratio.

In the light emitting device of this embodiment, the optical waveguide has an optical waveguide function even for yellow light. FIGS. 5A and 5B show the results of calculating the light distribution in the stacking direction of the semiconductor multilayer film 102 for the light with a wavelength of 440 nm and the light with a wavelength of 570 nm by the transfer matrix method, respectively. The phosphor layer 105 is a layer in which cylinders having r / a of 0.25 are arranged. For this reason, in the calculation, an effective volume filling factor was 55.5%, and an approximation was made as a uniform layer having an average refractive index of 1.56. In the light emitting device of this embodiment, Al _0.8 In _0.2 N having lattice matching with GaN but having a refractive index of 2.2 and a refractive index of GaN of 0.3 is used as the n-type cladding layer 121. For this reason, as shown in FIG. 5, it is possible to confine light strongly with respect to the stacking direction of the semiconductor multilayer film 102 at both of the wavelength 440 nm and the wavelength 570 nm. When the radiation angle from the end face of the optical waveguide is calculated based on the light distribution in the stacking direction shown in FIG. 5, the full width at half maximum θv of the far field distribution in the vertical direction is about 54 ° when the wavelength is 440 nm. In the case of 570 nm, it was about 50 °. This value indicates that the radiation is sufficiently narrow compared with a normal LED.

Further, when the refractive index difference Δn in the horizontal direction was calculated by the equivalent refractive index method, it was 5.06 × 10 ^{−3 at} the wavelength of 440 nm and 1.10 × 10 ^{−2 at} the wavelength of 570 nm. When the width of the optical waveguide is 4 μm and the full width at half maximum θh of the far-field distribution in the horizontal direction is calculated based on the obtained Δn value, it is about 6 ° for the wavelength 440 nm and about 7 for the wavelength 570 nm. It became °. This value indicates that the emission is very narrow compared to a normal LED.

  In the present embodiment, the phosphor layer 105 is arranged on the region serving as the optical waveguide so as to have a constant two-dimensional (refractive index) periodic structure. However, as shown in FIG. 6, the phosphor layer 105a formed on the central portion 109a of the optical waveguide and the phosphor layer 105b formed on the outer edge portion 109b may be arranged to have different periodic structures. . In FIG. 6, the phosphor layer 105a is arranged in a triangular lattice shape having a diameter 2r of 128.5 nm and a period a of 257 nm in the central portion 109a of the waveguide having a width of about 2.8 μm. . On the other hand, in the region having a width of about 0.8 μm, which is the outer edge portion 109b of the optical waveguide, a phosphor layer 105b in which openings 105c having a diameter of 210.7 nm are formed in a triangular lattice shape having a period a of 257 nm is formed. Yes. In any triangular lattice, the M points are arranged in the first Brillouin zone so as to coincide with the stripe direction.

  FIG. 7 shows the result of calculating the photonic band structure at the outer edge 109b by calculation. As shown in FIG. 7, in the outer edge portion 109b, a photonic band gap for TE polarized light exists in a range where a / λ is about 0.4 to 0.5. For this reason, when a is 257 nm, TE-polarized guided light is totally reflected at any angle on the outer edge 109 b of the optical waveguide in the range of λ from 514 nm to 642 nm. Therefore, the TE-polarized yellow light emitted from the phosphor layer 105 is confined inside the optical waveguide 109 and hardly leaks to the side of the optical waveguide 109. As a result, the luminous efficiency for yellow light can be further improved.

  The phosphor layer 105a formed at the central portion 109a and the phosphor layer 105b formed at the outer edge portion 109b have different shapes, thereby forming different two-dimensional periodic structures at the central portion 109a and the outer edge portion 109b. An example is shown. However, the period a of the phosphor layer 105a, which is a basic unit forming a two-dimensional periodic structure, and the period a of the phosphor layer 105b may be different from each other, and the phosphor that is a basic unit constituting the period. The radius r of the layer 105a and the radius r of the phosphor layer 105b may be different from each other. Moreover, the structure from which both the period a and the radius r mutually differ may be sufficient.

  In the present embodiment, the invention has been described using a triangular lattice in which a photonic band gap is easily formed. However, the two-dimensional periodic structure is not limited to a triangular lattice, and any periodic structure can be formed as long as a predetermined photonic band gap can be formed. It may be. Specifically, a square lattice, an oblique lattice, or the like may be used.

  FIG. 8 shows an example in which the light emitting device 200 of the present embodiment is used for the backlight of the liquid crystal panel 210. The light emitted from the light emitting element 200 travels inside the light guide plate 201, is emitted in a predetermined direction, and enters the total reflection prism 202. The light refracted in the direction perpendicular to the liquid crystal panel by the total reflection prism 202 passes through the polarizing plate 211 on the incident side, the liquid crystal panel 210 and the polarizing plate 212 on the outgoing side.

  Since the conventional LED has a full width at half maximum of the radiation angle of about 120 °, the coupling efficiency between the LED and the light guide plate is low. On the other hand, the light-emitting element of this embodiment has a very narrow θh of about 6 ° to 7 ° and a θv of about 50 ° to 54 °. Therefore, if the horizontal direction of the light emitting element 200 is aligned with the vertical direction of the light guide plate 201 and the vertical direction of the light emitting element 200 is aligned with the horizontal direction of the light guide plate, the light emitting element 200 and the light guide plate 201 The coupling efficiency can be increased. In addition, light can be diffused over a wide range within the surface of the light guide plate 201.

  In the case of an LED, about 50% of the generated white light is removed by the polarizing plate provided on the incident side of the liquid crystal panel. However, since the light-emitting element 200 of the present embodiment has a high TE polarization ratio, if the polarization direction transmitted through the polarizing plate is aligned with the TE polarization direction of the light-emitting element 200, less components are removed by the polarizing plate 211, and light is emitted. Utilization efficiency can be increased.

  FIG. 9 shows an example in which the light emitting element 300 of this embodiment is used as a light source of a projector. The light emitted from the light-emitting element 300 is converted into parallel light by the collimator lens 301 and then passes through the incident-side polarizing plate 311, the liquid crystal panel 310, and the outgoing-side polarizing plate 312. The transmitted light is magnified by the optical system 315 and projected onto the screen 316.

  The conventional LED has a large full width at half maximum of the radiation angle of about 120 ° and a large light radiation area. For this reason, it is necessary to use a large lens for the collimating lens. However, the light emitting element of this embodiment has a maximum radiation angle of about 50 ° to 54 ° and a small light emission area. For this reason, even if the collimating lens 301 is made small, light can be collimated efficiently. In the case of an LED, about 50% of the generated white light is removed by the polarizing plate provided on the incident side of the liquid crystal panel. However, since the light emitting element 300 of the present embodiment has a high TE deflection ratio, there are few components removed by the polarizing plate 311 and the light utilization efficiency can be increased.

  In the present embodiment, a white light emitting element using a blue SLD made of a GaN-based semiconductor multilayer film and a yellow phosphor made of YAG: Ce has been described. However, it is not limited to this, and other forms and materials may be used. For example, a combination of a blue laser diode made of a GaN-based semiconductor multilayer film and a green and red phosphor, or a combination of an ultraviolet SLD made of a GaN-based semiconductor multilayer film and a blue, green, and red phosphor. The same technique can be used when forming a white light emitting element.

  Further, the present invention is not limited to a white light emitting element, and can be applied to the purpose of controlling the polarization direction of light emitted from a phosphor in a light emitting element in which a waveguide type light emitting element and a phosphor are integrated. Therefore, the semiconductor multilayer film is not limited to the GaN system, but can be applied to a combination of a phosphor with a red or infrared light emitting element using a semiconductor multilayer film such as an AlInGaP system or an AlGaAs system. Further, the phosphor may be not only a material in which an oxide is doped with a rare earth element, such as YAG: Ce, but also a polymer or oxide glass in which semiconductor nanoparticles made of an organic dye, ZnS, CdSe, or the like are dispersed. Good.

  The light emitting element according to the present invention has high utilization efficiency of emitted light when used in a light source device, and is particularly useful as a light source for backlights, projectors and the like.

DESCRIPTION OF SYMBOLS 101 Substrate 102 Semiconductor multilayer film 103 Current confinement layer 104 Transparent electrode 105 Phosphor layer 105a Phosphor layer 105b Phosphor layer 105c Opening 107 p electrode 108 n electrode 109 Optical waveguide 109a Central portion 109b Outer edge 121 n-type cladding layer 122 Active Layer 123 p-side light guide layer 125 p-type contact layer 200 light emitting element 201 light guide plate 202 total reflection prism 210 liquid crystal panel 211 polarizing plate 212 polarizing plate 300 light emitting element 301 collimating lens 310 liquid crystal panel 311 polarizing plate 312 polarizing plate 315 optical system 316 screen

Claims (5)

A semiconductor multilayer film having an active layer formed on the main surface of the substrate and generating light of a first wavelength;
A plurality of phosphor layers formed on the semiconductor multilayer film and constituting a first two-dimensional periodic structure;
The phosphor layer is excited by the light of the first wavelength to generate light of the second wavelength,
The semiconductor multilayer film has an optical waveguide through which the light of the first wavelength and the light of the second wavelength are guided,
The first wavelength light and the second wavelength light emitted from the end face of the optical waveguide have a higher proportion of light in a horizontal direction than light in a direction perpendicular to the main surface of the electric field. A light emitting device characterized by the above.   The first two-dimensional periodic structure is characterized in that a photonic band gap is formed with respect to light whose direction of electric field is perpendicular to the main surface of the light having the second wavelength. The light emitting device according to claim 1. A phosphor layer formed in a central portion of the optical waveguide among the plurality of phosphor layers constitutes the first two-dimensional periodic structure,
The phosphor layer formed on the outer edge of the optical waveguide among the plurality of phosphor layers constitutes a second two-dimensional periodic structure,
3. The first two-dimensional periodic structure and the second two-dimensional periodic structure are different from each other in size or shape of a basic unit forming the period or the periodic structure. The light emitting element of description.   The second two-dimensional periodic structure is characterized in that a photonic band gap is formed with respect to light whose direction of electric field is parallel to the main surface of the light having the second wavelength. The light emitting device according to claim 3.   The light emitting device according to claim 1, further comprising a transparent electrode formed between the semiconductor multilayer film and the phosphor layer.

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